The field of art to which this invention pertains is aryl cyanate esters, i.e., cyanic acid esters of polyhydric phenols.
Industry needs lighter, stronger and more resistant materials to be used in place of the materials used today. For example, the aerospace industry is devoting considerable effort to utilizing structural composites in place of metals. Structural composites based on thermoplastic and thermoset resins and glass or carbon fibers have been and are being used successfully in many parts of military and commercial aircraft. The electronics industry needs materials which have better moisture and heat resistance and lower dielectric constants then those which have been used in the past. Thermoset resins finding use in such applications are based on epoxy resins, bismaleimide resins, and cyanate ester resins.
Materials which are flame resistant or fire retardant are needed for many of the old and new applications. Most cyanate esters, epoxy resins and bismaleimide resins are not flame resistant. The addition of other materials, such as phosphorus or bromine containing resins, is needed to obtain this property. Such modifications decrease thermal stability and glass transition temperature, and can adversely affect elevated temperature modulus, and dielectric loss properties as well as generate ionic species which cause metal corrosion.
Thermoset resins based on the dicyanate ester of 4,4'-(hexafluoroisopropylidene)bisphenol are inherently flame retardant and self-extinguishing. However, such resins have poor long term moisture resistance and poor caustic resistance. Structures based on such resins will absorb moisture from air having a relative humidity as low as 50 percent. When suddenly heated, the moisture can be expelled under forces sufficient to fracture the structure. The absorbed moisture also plasticizes the resin, thereby lowering its Tg and reducing the temperature at which it retains its stiffness.
In the processing of electronic parts, e.g., printed circuit boards, the parts are often subjected to aqueous caustic baths. Structures made with a matrix of the cyanate ester of 4,4'-(hexafluoroisopropylidene) bisphenol are degraded when contacted with the caustic.
Dicyanate esters made from 2,2-bis(4'-hydroxyphenyl-1,1,1, 3,3,3-hexafluoropropane) are disclosed in U.S. Pat. No. 4,157,360. Such dicyanate esters, referred to as 4,4'-(hexafluoroisopropylidene) bisphenol dicyanate, are described in detail in U.S. Pat. No. 4,745,215. Such dicyanate ester resins are said to have low dielectric constants, very low moisture absorption, good thermal stability and a high glass transition temperature (Tg). In addition, such resins are said to readily form alloys with both thermoplastic and thermosetting resins thereby allowing them to be used for a variety of electronic applications, i.e., packaging or circuit boards.
There is a need for cyanate ester compositions which not only are flame resistant but have moisture resistant and caustic resistant properties superior to the dicyanate ester of 4,4'-hexafluoroisopropylidene) bisphenol.